Not Applicable.
Not Applicable.
The present invention relates generally to sensors and, more particularly, to sensors having Hall cells for detecting rotations of a ferrous gear.
As is well known in the art, integrated gear tooth sensors using Hall cells and rotating targets are widely used in electronic automotive systems and many other such systems. In typical applications where an accurate detection of the tooth position is required, the Hall cell output is processed by relatively sophisticated electronics in order to cancel various distortive effects, such as offsets, temperature changes, airgap variations, production tolerances, and the like.
As shown in the prior art arrangement of FIG. 1, an integrated circuit sensor 10 can include a Hall cell 12 in proximity to a ferrous gear 14. The Hall cell 12 is attached to one pole face, e.g., N, of a magnet 16. The gear 14 rotates in close proximity to the Hall cell 12 such that the relatively strong field 18 of the magnet 16 or xe2x80x9cbackground fieldxe2x80x9d is weakly modulated by the airgap variations corresponding to the alternating tooth/valley perimeter of the rotating gear 14. The resultant signal variations reflect the gear 14 rotations as the Hall cell 12 generates a relatively small alternating signal voltage vsg embedded in the undesirable, relatively large direct current (DC) background signal Vbf, as shown in FIG. 2.
As is well known in the art, as much of the background signal Vbf as possible is removed for accurate processing of the signal vsg generated by rotation of the ferrous gear 14. Since the gear rotating frequency can vary within large limits including DC values, removing the background field signal Vbf by frequency discrimination is generally not practical.
FIGS. 3A-C show a common conventional sensor 20 arrangement for canceling the background field Vbf. In general, the prior art sensor 20 includes first and second Hall cells 12a, 12b coupled in a differential arrangement as shown in FIG. 3B. A subtractor 22 receives signals from the first and second Hall cells 12a,b and outputs the difference signal Vsg between the two signals. If the Hall cells 12a,b are properly spaced as compared to the tooth spacing on the gear 14, the difference voltage Vsg between the two Hall cells 12a,b produces the desired signal variations while the background field Vbf is cancelled for the most part. Due to assembly imbalances and Hall cell offsets, the signal at the subtractor 22 output includes a residual offset Vroff (FIG. 3C) that can reach five to ten percent or more of the background signal Vbf. To further reduce the residual offset Vroff, known sensors require additional sophisticated electronic processing or costly trimming techniques.
It would, therefore, be desirable to overcome the aforesaid and other disadvantages by providing a sensor having a Hall cell that eliminates the need to correct for assembly imbalances and associated offsets. It would further be desirable to provide a sensor that is more economical than conventional sensor arrangements.
The present invention provides a Hall sensor with a subtractor circuit that removes a magnetic background field from a signal that is modulated by airgap variations from a rotating ferrous gear. With this arrangement, the need for differential Hall cells is eliminated. While the invention is primarily shown and described in conjunction with a Hall cell detecting rotations of a toothed ferrous wheel, it is understood that the invention is applicable to Hall sensors in general in which it is desirable to remove a background signal.
In one aspect of the invention, a sensor includes a Hall cell and a compensation circuit providing respective inputs to a subtractor circuit. The Hall cell output signal provided to the subtractor circuit includes a first component corresponding to a background field and a second component corresponding to air gap variations detected by the Hall cell. The compensation circuit output signal to the subtractor circuit cancels the background field signal component in the Hall cell output signal. Thus, the remaining signal corresponds to the second component of the Hall cell output, i.e., the detected air gap variations, which can be generated by teeth on a rotating ferrous gear.
In one particular embodiment, the compensation circuit includes a comparator circuit receiving an amplified subtractor output signal and a threshold signal. The comparator output is gated with a clock signal and the output is provided to a counter having outputs coupled to a digital-to-analog converter (DAC), which outputs an analog signal to the subtractor. The counter increments until the comparator output stops the clock signal from changing the counter value. After initialization, the signal from the DAC to the subtractor increases until reaching a steady state corresponding to the background field as determined by the comparator. The subtractor then outputs the signal corresponding to gear rotation with the background field signal removed.